1. Field
Apparatuses and methods consistent with exemplary embodiments relate to a backlight unit which is capable of performing local dimming, and more particularly, to a backlight unit which is capable of performing local dimming using a surface light source, such as an Organic Light Emitting Diode (OLED).
2. Description of Related Art
Liquid Crystal Displays (LCDs) are one of the most widely used flat panel displays. An LCD is a device which displays an image using a display panel having two substrates on which electrodes are arranged and a liquid crystal layer interposed therebetween. For example, a transmissive type LCD displays an image by modulating light emitted from a backlight unit by controlling an electric field applied to the liquid crystal layer.
The image quality of the LCD depends on a contrast characteristic of the LCD. In order to enhance the contrast characteristic, a backlight dimming control method for adjusting the brightness of a backlight unit according to the displayed image has been developed. The backlight dimming control method can reduce power consumption by (adaptively adjusting the brightness of the backlight unit according to an input image. The backlight dimming method may be divided into a global dimming method for adjusting the entire brightness of a display surface, and a local dimming method for locally adjusting the brightness of a display surface. The global dimming method can enhance dynamic contrast, which is measured between a previous frame and a next frame. The local dimming method can enhance static contrast, which is difficult to improve in the global dimming method, by locally controlling the brightness of the display surface within a single frame period.
Backlight units are largely divided into direct type backlight units and edge type backlight units. Edge type backlight units have a structure in which a light source is disposed to face the side surface of a light guide panel and a plurality of optical sheets are arranged between an LCD panel and the light guide panel. Edge type backlight units can have smaller thickness than that of direct type backlight units due to their structural differences. However, in edge type backlight units, the light source irradiates light toward one side of the light guide panel and the light guide panel converts a linear light source or a point light source into a surface light source. Therefore, due to the basic structure of edge type backlight units, the light is diffused from the light guide panel and thus it is difficult to locally control the brightness. Thus, it is difficult to implement local dimming.
Compared with edge type backlight units, direct type backlight units have a structure in which a plurality of optical sheets and a diffusion plate are stacked under an LCD panel and a plurality of light sources are arranged under the diffusion plate. Direct type backlight units have the plurality of light sources arranged under the diffusion plate in a matrix formation, and enhance contrast by turning on the light source for a bright image area and turning off or turning down the light source for a dark image area.
Due to cost and heat limitations, the number of light sources to be arranged in a direct type backlight unit is limited. Because the light sources vertically rise and project light onto a transmissive type display, direct type backlight units have good light efficiency. However, because a direct type backlight unit should maintain a regular distance between the light sources for the sake of uniformity of the screen, direct type backlight units of the related art may suffer a blooming phenomenon in which an image blurs, and also, may have low contrast on a side surface since the light sources may interfere with an optical film in the backlight unit and thus light may leak.